A company wishing to provide telephone service to the members of the company may utilize a private branch exchange (PBX). Each telephone set that connects to and is served by the PBX is referred to as a client station or station. The use of a PBX may help to avoid the burden and cost of separately connecting each of the company's telephone sets to the public switched telephone network (PSTN). In addition, a PBX may provide additional advanced features which may not be achievable by connecting the stations directly to the PSTN. For example, the PBX may provide improved privacy when calling between stations, since conventional calls on the PSTN are transmitted across a public network, which is subject to eavesdropping. In addition, the PBX may provide additional services, such as call park, call pickup, call transfer, and call forward to other stations.
A PBX may be viewed as having two sides: a private side where the PBX exchanges signaling and media information with its client stations, and a public side where it connects to the PSTN to exchange signaling and media information with the telephone company. The PBX's public connection with the PSTN is typically referred to as a trunk, and the public side is typically referred to as the trunk side. In a traditional PBX system, the trunk may be implemented as a T1 or T3 line.
Voice Over Internet Protocol (VoIP) has seen increased widespread usage. Session Initiation Protocol (SIP) is an application-layer control protocol that can establish, modify, and terminate multimedia sessions such as Internet telephony calls. SIP is defined in RFC-3261, “SIP: Session Initiation Protocol,” which is incorporated by reference herein in its entirety. An IP PBX is a type of PBX that connects to client stations on the private side via an IP network, and connects to an Internet Telephone Service Provider (ITSP) on the public side via an IP network. The ITSP includes PSTN gateways, which provide PSTN termination services. Where SIP is used as the signaling protocol between the IP PBX and the ITSP, the logical connection between the IP PBX and the ITSP is referred to as a SIP trunk. The IP PBX may route SIP calls received from the ITSP to the target station in the IP PBX's SIP network. Voicemail services for the client stations may also be provided by the ITSP or from a separate voicemail service provider, such as an Internet Voice Mail Service Provider (IVMSP).
The IP PBX typically serves multiple extensions and may be associated with multiple Direct Inward Dial (“DID”) numbers. It may be desirable to provide call routing features in an IP PBX that enable the IP PBX to route incoming calls directed to a particular telephone number to one or more extensions based on definable rules.